Heretofore, many salt hydrates and their eutectics have been identified which are useful in storing thermal energy for environmental heating or cooling use. Such compounds generally have melting points ranging between 40 degrees Fahrenheit and 120 degrees Fahrenheit and have a heat of fusion in excess of 50 BTU per pound. The most well known of these compounds is sodium sulfate decahydrate (Na.sub.2 SO.sub.4.10H.sub.2 O).
When such a compound is sealed in a relatively thin walled plastic or elastomeric container, some water in the liquid phase of the compound is eventually lost by diffusion through the walls of the container, reducing the energy storage efficiency of the compound.
Another phenomenon which reduces the energy storage efficiency of a salt hydrate compound is vertical separation of the compound due to incongruent melting in which, for example, in the case of sodium sulfate decahydrate, some anhydrous sodium sulfate crystals are formed which sink to the bottom of the container and are separated from water at the top of the container by a layer of sodium sulfate decahydrate crystals formed therebetween. One solution to this problem has been the provision of elaborate means for mechanical mixing or stirring. Another solution proposed has been the use of a clay-like homogenizing agent such as magnesium aluminum silicate. It is known that this vertical separation phenomenon does not markedly affect the energy storage efficiency of the salt hydrate if the effective vertical height of the container is no more than about one-half inch.
Another difficulty which has existed concerning the use of salt hydrate for thermal energy storage is that no suitable container therefor has been available which may be housed within walls, ceilings or floors of a building.